Democratic governments regularly supply weapons to what are sometimes called “outlaw states” – oppressive regimes that violate the basic rights of their own citizens, or aggressive regimes that wrongfully threaten the security of outsiders. Sometimes democratic governments sell the weapons themselves; sometimes they issue export licenses to private arms firms within their jurisdiction.

Both practices are frequently condemned on moral grounds. But how might governments who help to arm outlaw states try to defend themselves? What arguments could they appeal to in an attempt to justify their actions?

Politicians sometimes claim that their acts make no difference to the degree of suffering inflicted by the regimes that they arm – that if they didn’t sell weapons to the regimes in question, some other government would. For example, when it was revealed in 2014 that Hong Kong’s riot police had used British-made tear gas against unarmed pro-democracy protesters, then foreign secretary Philip Hammond remarked: “CS gas is available from large numbers of sources around the world. To be frank, I think that is a rather immaterial point. They could buy CS gas from the US.”

But as I argue in an article for the journal Political Studies, this kind of argument has multiple shortcomings. When a government permits its firms to compete for certain customers in the international arms market, it puts downward pressure on prices, and this could allow outlaw or oppressive states to purchase weapons in larger quantities. Some governments also offer certain kinds of weapons, or weapons of a particularly high quality, that recipients would not be able to acquire from elsewhere. By offering these weapons, governments increase the efficiency with which their trading partners can pursue their unjust ends.

In addition, the intentions of other suppliers should not be regarded as inalterable features of the world. After all, this is not generally how governments regard the intentions of their foreign counterparts. If it were, diplomacy would be dismissed as a waste of time and resources. When a government wants another to change its behaviour, it tries to persuade it to do so.

If the British government thought it would be better if the Hong Kong authorities lacked easy access to tear gas, it could have impressed its rationale upon its American allies, rather than simply taking the Americans’ behaviour as a given and then trying to exploit that behaviour as a justification for its own.

The enemy’s enemy

As I’ve tried to demonstrate in recent work, while many of the arguments that governments employ to defend the sale of weapons to outlaw states are weak and self-serving there is one that, when applicable, has potentially greater force.

Sometimes, arms transfers to an oppressive regime can reasonably be expected to actually reduce the degree of oppression that is inflicted. Arms transfers can do this when they help a regime to repel an even more oppressive rival that threatens to overthrow it. Consider the US Lend-Lease programme, which sanctioned arms transfers to the Soviet Union during World War II. This policy could be defended on the plausible grounds that arms transfers enabled the Red Army to resist the greater oppression that would otherwise have been imposed by the Nazis.

Of course, situations like this don’t arise often – and even when they do, arms transfers are not necessarily justifiable.

Evaluation of a proposed arms transfer to an outlaw state must take a comparative form. Whether the transfer can be justified depends on how it fares compared to other actions that could be taken instead. If the transfer is expected to produce worse outcomes than alternative available options, then it isn’t morally acceptable.

Different kinds of intervention or assistance have to be evaluated on a case-by-case basis, but arms transfers always come with serious problems. Most obviously, they provide outlaw states with tools that can be used for oppressive and aggressive ends (in addition to any legitimate defensive ends): other types of support lack this feature. Then there’s the problem of “leakage”. Outlaw states may pass on weapons to third parties, or be unable to ensure the security of stockpiles. The risk of stockpiles being looted is especially high in times of crisis.

In short, supplying weapons to outlaw states is difficult to justify even under the most favourable circumstances. If democratic politicians care about the ethical status of their acts, and aspire to conduct themselves in a manner that can be justified to others, then they should take this fact seriously, and end their casual, callous promotion of trade with tyrants.